Evidence for functional adenosine A3 receptors in microglia cells

Adenosine exerts its effects through four subtypes of G-protein-coupled receptors (GPCRs): adenosine A(1) and A(3) receptors (A(3) R), which generally couple to G(i) proteins and adenosine A(2A) and A(2B) receptors that activate G(s) proteins. Though there is evidence for the expression of mRNA for the A(3) R in the central nervous system, evidence for functional receptors has depended on drugs with uncertain specificity. Here, we show that A(3) Rs mediating functional responses are present in microglia cells. By selectively stimulating the A(3) R in both primary mouse microglia cells and the N13 microglia cell line with the agonist Cl-IB-MECA, we have found a biphasic, partly G(i) protein-dependent influence on the phosphorylation of the extracellular signal-regulated protein kinase 1/2 (ERK1/2). ERK1/2 activation was assessed by immunoblotting with phospho-specific antibodies. The involvement of the A(3) R in Cl-IB-MECA-induced ERK1/2 phosphorylation was confirmed by demonstrating that those effects are absent in primary mouse microglia cells isolated from mice lacking the gene for the A(3) R.